fix handling of 0 bps values
fix handling of 0 bps values

Summary:
This fixes the fboss port details command to handle 0 bps
fields properly. We assert that bps is set, but since 0 is considered
falsy by python the assert fails.

Differential Revision: D3042667

fb-gh-sync-id: ff0ae9e3d5f3c3109fc7bf27ba798f1c14e50a06
fbshipit-source-id: ff0ae9e3d5f3c3109fc7bf27ba798f1c14e50a06

peterlei/fboss,peterlei/fboss,peterlei/fboss,peterlei/fboss,neuhausler/fboss,neuhausler/fboss,neuhausler/fboss,neuhausler/fboss